This invention relates to the growth of epitaxial layers of II-VI semiconductor compounds, such as ZnSe, and more particularly relates to a method of incorporating N acceptors into the II-VI lattice of such compounds.
Co-pending parent application Ser. No. 894,308, describes a method of N-doping epitaxial layers of II-VI semiconductor compounds during growth by flow modulation epitaxy (FME) using NH.sub.3 as the dopant carrier.
One problem encountered when using NH.sub.3 as the carrier, is the tendency toward passivation of N acceptors due to the presence of H. This is due to the inability of NH.sub.3 to completely dissociate into N at the growth surface. This problem persists even if photolysis of the carrier is performed above the growth surface, probably due to the short free mean path between the dissociated species, giving rise to recombination of these species.
Another problem that arises when using NH.sub.3 as the carrier is that it tends to react with the carriers of the cation and anion species, due to its highly basic chemical nature. This results in degradation of the II-VI compound crystal lattice, such degradation increasing with increasing flow rate of the dopant carrier gas.
Tertiary Butyl Amine (TBNH.sub.2) has been used as the N dopant source in the epitaxial growth of ZnSe by molecular beam epitaxy (MBE). However, the TBNH.sub.2 was dissociated into (CH.sub.3)C--and --NH.sub.2 by thermal cracking at temperatures in the range of about 550 to 850 degrees C prior to being introduced into the growth chamber. S. Zhang and N. Kobayashi, Jpn. J. Appl. Phys., 31, L666 (June 1992). Thus, the above-described problems of passivation and reactivity encountered when using NH.sub.3 would be expected to be encountered in this technique as well.
When epi layers of ZnSe are grown by the technique of MOVPE (metal organic vapor phase epitaxy), photo-assisted growth using above bandgap illumination (illumination whose energy is above the bandgap energy of the semiconductor compound at the growth temperature) has been found to enhance the growth rate of the layer, permitting lower growth temperatures, at which the sticking coefficient of the N species is increased. Sg. Fujita et al, Jpn. J. Appl. Phys., 26, L2000 (1987). In addition, the use of below-bandgap illumination has been observed to enhance doping efficiency on the Se sublattice. Sz. Fujita et al, J. Cryst. Grow., 101, 48 (1990).